In the petroleum refinery process for producing various fuel oil such as gasoline, diesel, kerosene, etc., the sulfur in the crude oil is removed as hydrogen sulfide gas by hydrodesulfurization process. The highly toxic hydrogen sulfide gas is then converted to elemental sulfur in sulfur-recovery plants or so-called Claus plants. During the last two decades, a great number of Claus tail-gas treating (TGT) processes have been developed to increase the total sulfur-recovery efficiency. Conventional Claus TGT processes involve a hydrogen sulfide absorption step, in which a tail gas containing unreacted hydrogen sulfide is introduced into an alkaline bath. Removing the last percentages of sulfur by means of these conventional Claus TGT processes is relatively expensive, both in terms of capital investment cost and energy consumption.
Recently, in order to avoid the shortcomings of these solution-absorption type Claus TGT processes, two dry types of TGT processes have been developed, that is, Mobil-direct-oxidation process developed by Mobil AG Company in Germany (Oil and Gas Journal, 86, p.63-67, 1988) and Super-Claus Process developed by Comprimo Company in Netherlands (Catalysis Today, 16, p263-271, 1993), both of which comprise a step of recovering elemental sulfur from Claus tail gas by selective oxidation of hydrogen sulfide in the presence of a catalyst. The catalyst used in Mobil-direct-oxidation process contains titanium dioxide (TiO.sub.2). The catalyst used in Super-Claus Process is an active mixture of iron and chromium oxides deposited on an alpha-alumina support. These dry type Claus TGT processes are simple and economical; however, the chromium atom contained in the catalyst is a toxic substance .
In our experimental studies, it was found that vanadium and magnesium mixed catalyst can effectively oxidize hydrogen sulfide to elemental sulfur (U.S. Pat No. 5,653,953 and Taiwanese Patent Published No. 92615). However, as the content of magnesium in the mixed catalyst is increased, the yield of sulfur will be significantly reduced.
Therefore, a major object of the present application is to improve the defects encountered with the prior arts.